The invention relates to laser systems, and more particularly, to techniques for mounting an optical bench in a chassis, and for managing thermal and mechanical stresses associated with the bench.
Generally, all lasers require a degree of thermal and mechanical stability to maintain output power, beam divergence, and mechanical boresight. Stability requirements are even higher in demanding applications, such as surveying and targeting. In particular, laser design involves balancing laser performance requirements against stability requirements associated with the laser""s intended application, as well as balancing ease of manufacture and laser alignment complexity against stability requirements.
Ideally, lasers should be designed and manufactured with no adjustable components. With everything fixed, laser alignment would depend solely on the quality of the basic design. There would be no possibility of misalignment in the field. However, designs with no adjustable components (e.g., mounts) produce lasers with relatively broad tolerances and poor performance. Such designs are therefore not practical for demanding applications. Laser adjustment systems have been developed that result in acceptable laser alignment, but only at the cost of increased complexity and laser alignment labor. This problem is amplified for systems having high stability requirements, such as military systems.
In addition to alignment requirements, output beam boresight stability over a range of environmental and operational conditions must also be considered in laser design. A primary cause of poor boresight stability is mechanical motion of optical bench mounted components due to thermal loads caused not only by the surrounding environment, but also by the laser system itself.
In more detail, as internal temperature changes occur in a laser system, each of the system""s components expands or contracts at its corresponding rate of thermal expansion. Neighboring components having different coefficients of thermal expansion are therefore subjected to differential thermal expansion. This differential thermal expansion, when left uncompensated, results in component movement as the system is exposed to temperature variation.
As such, the system""s output beam deviates laterally and angularly, thereby changing the boresight alignment, resulting in an unreliable laser. In applications such as surveying, targeting, countermeasures, and alignment of machinery and buildings, such boresight instability is unacceptable. Yet, typical laser system designs fail to provide a comprehensive thermal load management scheme that systemically resolve tensions caused by differential thermal expansion. This has resulted in laser systems having inferior thermal stability ratings.
What is needed, therefore, are techniques for mounting and designing optical benches for laser systems that minimize the effects of thermal stress and thereby maintain output power, beam divergence and, in particular, mechanical boresight of the system""s laser.
One embodiment of the present invention provides a system for mounting a laser""s optical bench to a chassis. The system includes a first pseudo semi-kinematic sub-mount that is adapted to constrain the optical bench in at least one translational direction, a second pseudo semi-kinematic sub-mount that is adapted to constrain the optical bench in two translational directions, and a third pseudo semi-kinematic sub-mount that is adapted to constrain the optical bench in at least two translational directions. The first, second, and third pseudo semi-kinematic sub-mounts operate together to form a pseudo semi-kinematic mount that constrains the optical bench in six degrees of freedom, without significant over constraint.
In one such embodiment, the first pseudo semi-kinematic sub-mount constrains the bench in one translational direction, and the third pseudo semi-kinematic sub-mount constrains the bench in three translational directions. For example, the first pseudo semi-kinematic sub-mount constrains the bench in the Z translational direction, the second pseudo semi-kinematic sub-mount constrains the bench in the X and Z translational directions, and the third pseudo semi-kinematic sub-mount constrains the bench in the X, Y and Z translational directions. Alternatively, the first pseudo semi-kinematic sub-mount constrains the bench in two translational directions, and the third pseudo semi-kinematic sub-mount constrains the bench in two translational directions. For example, each of the first, second, and third pseudo semi-kinematic sub-mounts constrains the bench in the X and Y translational directions.
One or more of the pseudo semi-kinematic sub-mounts may include a first optical bench mounting pad in surface contact with a first chassis mounting pad so that one or both of the first pads can slide with respect to the other. The sub-mount may further include a second mounting pad that is perpendicular to the first pads for providing a second translational constraint. Likewise, the sub-mount may further include a third mounting pad for providing a third translational constraint. One or more of the pseudo semi-kinematic sub-mounts may include a flexure device that constraints the optical bench in two or more translational directions and provides at least one degree of rotational freedom. One or more of the pseudo semi-kinematic sub-mounts may include a spring-loaded bolt fed through a clearance hole or slot and secured in a threaded hole. One or more of the pseudo semi-kinematic sub-mounts may include a Teflon shim to reduce friction between contacting surfaces.
Another embodiment of the present invention provides a system for mounting a laser""s optical bench to a chassis. This particular system includes a first pseudo semi-kinematic sub-mount including a first optical bench mounting pad in surface contact with a first chassis mounting pad so that at least one flat contact surface of one or both of the first pads can slide with respect to the other. The system further includes a second pseudo semi-kinematic sub-mount including a second optical bench mounting pad in surface contact with a second chassis mounting pad so that at least one flat contact surface of one or both of the second pads can slide with respect to the other, and a third pseudo semi-kinematic sub-mount that is adapted to constrain the optical bench in at least two translational directions.
Another embodiment of the present invention provides a method for assembling a laser system including an optical bench adapted for mounting in a chassis. The method includes constraining the optical bench in the chassis in at least one translational direction with a first pseudo semi-kinematic sub-mount, constraining the optical bench in the chassis in two translational directions with a second pseudo semi-kinematic sub-mount, and constraining the optical bench in the chassis in at least two translational directions with a third pseudo semi-kinematic sub-mount. The first, second, and third pseudo semi-kinematic sub-mounts operate together to constrain the optical bench in six degrees of freedom, without significant over constraint.
The method may further include mounting a pair of hot elements on an outside wall and on opposite faces of the optical bench to balance thermal load on the bench. The pair of hot elements might include, for example, a laser crystal, a compensating heat source, and a pump diode. The method may further include mounting a hot element on the optical bench in a thermally dammed area, thereby forcing heat load from the hot element to exit to a known area of the optical bench. A heat dumping mechanism may be provided at the known area to dissipate the heat load. The method may further include mounting a hot component of the optical bench on a cooling device (e.g., cold finger), and plugging the cooling device into a chassis hole and through a clearance hole in the optical bench. The method may further include providing a heat strap to transfer unwanted heat to a heat dump.
The features and advantages described herein are not all-inclusive and, in particular, many additional features and advantages will be apparent to one of ordinary skill in the art in view of the drawings, specification, and claims. Moreover, it should be noted that the language used in the specification has been principally selected for readability and instructional purposes, and not to limit the scope of the inventive subject matter.